Magnesium alloy chips and process for manufacturing molded article using same

ABSTRACT

There is provided chips for injection molding wherein the surfaces of chips made of an aluminum-containing magnesium alloy are coated with carbon powder. A molded article produced by injection molding of such chips for injection molding had excellent bending properties and tensile strength, which vary in a small range. Furthermore, a scrap formed during injection-molding of the chips for injection molding has improved recyclability.

TECHNICAL FIELD

The present invention relates to magnesium alloy chips for injectionmolding and a process for manufacturing a molded article using same.

BACKGROUND ART

A magnesium alloy has a high specific strength since it is mostlightweight among practically used metals, and exhibits excellent heatradiation and better recyclability than a resin. Thus, molded articlesmade of a magnesium alloy are used in a wide variety of applicationssuch as electric devices, automobiles and leisure goods.

Injection molding is a common molding methods for a magnesium alloy. Ingeneral, a magnesium alloy is injection-molded by heating chips made ofa magnesium alloy in a cylinder into a molten or semi-molten state(coexistence of a solid and a liquid phases) and then injecting themolten or semi-molten magnesium alloy into a mold. Here, since themagnesium alloy is injected into a mold at a relatively higher pressure,injection-molding is preferable for forming a thin-wall article such asa casing of an electric device. A so-called thixomolding method, thatis, inter alia, a molding method in which a semi-molten material isinjected into a mold, is a typical injection molding method for amagnesium alloy and is used for producing various molded articles.

Conventionally, a Mg—Al alloy having excellent mechanical properties,particularly a Mg—Al—Zn alloy being well-balanced between mechanicalproperties and processability and exhibiting improved corrosionresistance have been widely used as a magnesium alloy for injectionmolding. Recently, further improvement in mechanical properties of amolded article has been needed in order to allow for thinning of amolded article made of a magnesium alloy and increasing an yield.

There has been known a method for adding carbon to an alloy for thepurpose of improving mechanical properties of a molded article made ofan aluminum-containing magnesium alloy. Addition of carbon to amagnesium alloy makes crystals finer, resulting in improved mechanicalproperties. It is believed that such reduction in a crystal size may becaused by Al₄C₃ formed by a reaction between carbon and aluminum whichare added to a magnesium alloy. Conventionally, carbon is added to amagnesium alloy by adding C₂Cl₆ to a molten magnesium alloy. Such amethod, however, has an environmental disadvantage that C₂Cl₆ added isdecomposed to generate harmful substances such as chlorine gas, and thusan alternative method has been needed.

A known alternative method for adding carbon to an aluminum-containingmagnesium alloy is addition of carbon powder (for example, see PatentReference Nos. 1 and 2). However, when carbon powder, is directly addedto a molten magnesium alloy, the carbon powder tends to agglutinate, sothat a molded article obtained may have insufficiently improved or havevaried mechanical properties.

Patent Reference No. 3 has described a process for manufacturing acarbon-containing magnesium alloy comprising mixing 5 to 30 parts byweight of at least one of carbon powder, carbon nanofiber and carbonnanotube with 100 parts by weight of a magnesium alloy to prepare amaster batch and then mixing the master batch with a 3- to 20-fold partsby weight of a magnesium alloy. In the examples therein, there isdescribed a magnesium alloy produced by processing magnesium alloypowder and carbon powder in a ball mill to prepare a mixed powder,sintering the mixed powder to prepare a master batch, adding the masterbatch to a molten metal and homogenizing the molten metal with stirring.There is described that the magnesium alloy thus produced containsuniformly dispersed carbon and has improved tensile strength and ahigher Young's modulus. The process is, however, troublesome and thus ata disadvantage in terms of cost.

PRIOR ART REFERENCES Patent References

-   Patent Reference No. 1: Japanese published unexamined application    No. 1994-73485.-   Patent Reference No. 2: Japanese published unexamined application    No. 2004-156067.-   Patent Reference No. 3: Japanese published unexamined application    No. 2007-291438.

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

To solve the above problems, an objective of the present invention is toprovide chips for injection molding allowing for forming a moldedarticle made of a magnesium alloy which has excellent bending propertiesand tensile strength which vary in a small range. Another objective ofthe invention is to provide a process for manufacturing a molded articlemade of a magnesium alloy using such chips for injection molding.

Means for Solving Problem

The above problems can be solved by providing chips for injectionmolding wherein the surfaces of chips made of an aluminum-containingmagnesium alloy are coated with carbon powder.

Herein, a content of the carbon powder is preferably 0.01 to 3% byweight. Furthermore, the carbon powder is preferably carbon black. Here,the carbon black more preferably has an average primary particlediameter of from 5 to 100 nm and a DBP absorption of from 40 to 200mL/100 g.

The above problems can be also solved by providing a process formanufacturing the chips for injection molding, comprising mixing chipsmade of an aluminum-containing magnesium alloy with the carbon powder.

A preferable embodiment of the present invention is a process formanufacturing a molded article made of a magnesium alloy, comprisingcharging the chips for injection molding in an injection-molding machineand then injection-molding the chips.

Here, in the molded article, a complex of aluminum and carbon isdispersed in a magnesium matrix.

Another preferable embodiment of the present invention is a process formanufacturing an ingot made of a magnesium alloy, comprisingheat-melting a scrap formed during injection-molding of the chips forinjection molding in the presence of a flux and then cooling it. Here,it is preferable that a ratio (C₂/C₁) of a carbon content C₂ (% byweight) in said ingot to a carbon content C₁ (% by weight) in said scrapis 0.1 or less.

Advantageous Effects of the Invention

A molded article produced by injection-molding of chips for injectionmolding of the present invention has excellent bending properties andtensile strength which vary in a small range. Furthermore, a process formanufacturing a molded article made of a magnesium alloy of the presentinvention can conveniently provide a molded article made of a magnesiumalloy having excellent bending properties and tensile strength whichvary in a small range. It thus allows for making a molded articlethinner and improving an yield. Furthermore, a scrap formed duringinjection-molding of the chips for injection molding has improvedrecyclability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exterior photo of a test piece in a tensile test and anexterior photo of a testing machine on which the test piece is set.

FIG. 2 shows an exterior photo of a testing machine on which the testpiece is set, in a bending test.

FIG. 3 shows micrograms of a cross section of a molded article cut in adirection vertical to a flowing direction of a molten metal in Example 1and Comparative Example 1.

FIG. 4 shows a relationship between a displacement and a load at breakof a test piece as determined by a tensile test in Example 1 andComparative Example 1.

FIG. 5 shows a relationship between a displacement and a load at breakof a test piece as determined by a bending test in Example 1 andComparative Example 1.

FIG. 6 shows an elemental map of an area containing a complex ofaluminum and carbon in the surface of a molded article in Example 1.

FIG. 7 shows measurement points in determination of content distributionof each of aluminum and zinc in a molded article in Example 1 andComparative Example 1.

FIG. 8 shows content distribution of aluminum in a molded article inExample 1 and Comparative Example 1.

FIG. 9 shows content distribution of zinc in a molded article in Example1 and Comparative Example 1.

FIG. 10 shows a 0.2% proof stress of a molded article in Examples 1 to 3and Comparative Example 1 as determined by a tensile test.

MODE FOR CARRYING OUT THE INVENTION

The present invention provides chips for injection molding wherein thesurfaces of chips made of an aluminum-containing magnesium alloy arecoated with carbon powder.

The chips to be coated with carbon powder must be made of analuminum-containing magnesium alloy. In other words, the chips must bemade of an alloy containing aluminum as a component in addition tomagnesium. Aluminum is effective for improving tensile strength andcorrosion resistance of a magnesium alloy. Furthermore, as described inExamples later, a complex of aluminum and carbon is formed in a moldedarticle produced by the present invention. Formation of the complexwould lead to a molded article having excellent bending properties andtensile strength.

An aluminum content in a magnesium alloy used in the present inventionis preferably 1 to 15% by weight. If an aluminum content is less than 1%by weight, tensile strength and corrosion resistance of a molded articleproduced may be reduced. Furthermore, formation of a complex of aluminumand carbon may be inhibited in a molded article produced, thus bendingproperties and tensile strength may not improve. An aluminum content ofmore than 15% by weight may lead to brittle failure.

The magnesium alloy can contain zinc, wherein a zinc content is 3% byweight or less. In a case that zinc is contained therein, toughness ofmagnesium alloy and fluidity during molding further improve. A zinccontent is preferably 0.1 to 3% by weight. If a zinc content is lessthan 0.1% by weight, toughness of a molded article produced and fluidityduring molding may be reduced. If a zinc content is more than 3% byweight, hot tearing may occur.

The magnesium alloy can contain manganese, in which a manganese contentis 1% by weight or less. In a case that manganese is contained therein,corrosion resistance of a magnesium alloy further improves. A manganesecontent is preferably 0.05 to 1% by weight. If a manganese content isless than 0.05% by weight, corrosion resistance of a molded articleproduced may be reduced. If a manganese content is more than 1% byweight, compression strength and tensile strength may be reduced.

The magnesium alloy can contain beryllium, in which a beryllium contentis 0.003% by weight or less. In a case that beryllium is containedtherein, flame resistance during melting of a magnesium alloy improves.In a case that beryllium is contained therein, brightness of a moldedarticle produced also improves. A beryllium content is preferably 0.0001to 0.003% by weight. If a beryllium content is less than 0.0001% byweight, it may fail to improve flame resistance or brightness. If aberyllium content is more than 0.003% by weight, crystals may be coarse,leading to reduction of tensile strength and a higher cost.

The magnesium alloy can contain calcium, in which a calcium content is3% by weight or less. In a case that calcium is contained therein, flameretardancy of a magnesium alloy improves. A calcium content is generally0.5 to 3% by weight.

The magnesium alloy can contain elements other than those describedabove as long as they do not reduce the effect of the present invention.Such elements can be willingly added or contained as inevitableimpurities. A content of such elements is generally 1% by weight orless. The balance of a magnesium alloy used for the chips is magnesium,whose content is generally 80% by weight or more.

The magnesium alloy can be specifically selected from magnesium alloyssuch as AZ91, AM50, AM60 and AZ31 in accordance with the ASTM Standard.Among others, AZ91 is preferable, which is well-balanced betweenmechanical properties and processability and exhibits higher corrosionresistance.

There are no particular restrictions to a process for manufacturing thechips. Generally, an ingot made of the above magnesium alloy can be cutinto the chips. There are no particular restrictions to the shape or thesize of the chips, which can be appropriately selected, depending on,for example, the specifications of an injection-molding machine used forproducing a molded article. Chips with a length of from 1 to 10 mm isgenerally used. Herein, a length of a chip denotes a distance betweenthe furthest positions in the chip.

The carbon powder used in the present invention can be selected from,but not limited to, carbon black, graphite such as scaly graphite, cokeor the like.

In the present invention, the carbon powder is preferably carbon black.When carbon black is used as the carbon powder, carbon black and thechips are mixed by, for example, a mixer to coat the surfaces of thechips with carbon black. The use of chips for injection molding coatedwith carbon powder in injection molding would allow carbon powder toeasily disperse in a magnesium alloy.

There are no particular restrictions to the type of the carbon black.Examples which can be used include furnace black, thermal black, channelblack, acetylene black ketjen black and the like, which can be used incombination.

Preferably, the carbon black has an average primary particle diameter offrom 5 to 100 nm and a DBP absorption of from 40 to 200 mL/100 g. A DBPabsorption is a parameter corresponding to a volume of voids in a bunchof grapes when fused primary particles of carbon black, so-called“aggregate”, is assumed to be a bunch of grapes. As the aggregate grows,the void volume increases and thus a DBP absorption increases. Formationof a complex of aluminum and carbon may be influenced by a primaryparticle diameter of carbon black and also a growing state of theaggregate. It is, therefore, preferable that an average primary particlediameter and a DBP absorption are within a certain range. A DBPabsorption can be determined in accordance with JIS K6217.

In the light of higher proof stress of a molded article produced, thecarbon black preferably has a DBP absorption of from 40 to 200 mL/100 g,more preferably 60 to 200 mL/100 g, further preferably 80 to 200 mL/100g.

The carbon black can have functional groups in its surface. Examples ofsuch functional groups include hydroxy groups such as phenolic hydroxygroup, carboxyl groups and quinone groups.

The surfaces of the chips are coated with the carbon powder to producechips for injection molding having surfaces coated with the carbonpowder. There are no particular restrictions to a method for coating thesurface of the chips with the carbon powder. Generally, the chips andthe carbon powder can be mixed using a mixer to produce chips forinjection molding having the surfaces coated with the carbon powder. Amixing ratio of the chips to the carbon powder can be appropriatelyadjusted, depending on the amount of carbon contained in a moldedarticle to be produced. The amount of the carbon powder in the chips forinjection molding coated with the carbon powder is preferably 0.01 to 3%by weight, more preferably 0.01 to 0.5% by weight.

The chips for injection molding having surfaces coated with the carbonpowder are charged in an injection-molding machine and theninjection-molded to provide a molded article. Generally, the chips forinjection molding charged in an injection-molding machine are heated ina cylinder while being fed via a screw in the cylinder to an injectionnozzle. Then, a molten or semi-molten (coexistence of a solid and aliquid phases) magnesium alloy fed to the vicinity of the injectionnozzle is injected into a mold to be shaped. In general, a cylindertemperature in an injection-molding machine is 530 to 700° C. and a moldtemperature is 160 to 240° C.

Thus, the use of the chips for injection molding having surfaces coatedwith the carbon powder allows the carbon powder to be homogeneouslydispersed in the molten or semi-molten magnesium alloy in theinjection-molding machine, so that a molded article in which a complexof aluminum and carbon is homogeneously dispersed is obtained. In theinjection-molding machine, it seems that a magnesium alloy molten orsemi-molten by heating could be so efficiently stirred by rotation of ascrew that the carbon powder can be homogeneously dispersed in themolten or semi-molten magnesium alloy. It is surprising that the carbonpowder is homogeneously dispersed in a magnesium alloy in spite that acylinder temperature is not so high and a time taken for injection aftercharging the chips in the cylinder. In the present invention, preferredis a molding process where chips introduced in an injection-moldingmachine are semi-molten and then injected in a mold, a so-calledthixomolding method.

In a molded article produced by a manufacturing process according to thepresent invention, a complex of aluminum and carbon is dispersed in amagnesium matrix. A complex of aluminum and carbon can be observed byelement mapping using, for example, an X-ray microanalyzer. In thecomplex region, aluminum and carbon are detected in higher levels thansurrounding regions. A magnesium matrix denotes regions other than thecomplex of aluminum and carbon, and the major part of the matrixcontains magnesium as a main component.

In the present invention, a complex of aluminum and carbon would beformed by bond formation between the carbon powder and aluminum in thechips during injection molding. Our analysis of a molded articleproduced have confirmed that a large part of carbon in the moldedarticle forms a complex with aluminum. Although it is not confirmedwhether Al₄C₃ is formed in the complex, formation of such a complexwould allow a molded article of the present invention to have excellentbending properties and tensile strength. Furthermore, according to themanufacturing process of the present invention, the carbon powder can behomogeneously dispersed in a magnesium alloy, so that the complex ishomogeneously dispersed in a molded article. Therefore, variation inbending properties and tensile strength in the molded article isreduced.

Furthermore, in a molded article of the present invention, defects arereduced and a segregation level for each component is low. It would bebecause fluidity is improved by dispersing carbon powder in a molten orsemi-molten magnesium alloy during injection molding. Reduction ofdefects and a lower segregation level also contribute to smallervariation in bending properties and tensile strength.

A carbon content in a molded article produced according to the presentinvention is preferably 0.01 to 3% by weight. If a carbon content isless than 0.01% by weight, bending properties and tensile strength ofthe molded article may be insufficiently improved and fluidity may beinsufficiently improved. If a carbon content is more than 3% by weight,carbon powder may agglutinate, leading to tendency to crack formationand thus causing variation in tensile strength. A carbon content is morepreferably 0.5% by weight or less.

A molded article thus produced has excellent bending properties andtensile strength which vary in a small range. A molded article can be,therefore, thinner and produced in an improved yield. A molded articleproduced by a manufacturing process of the present invention can besuitably used in a variety of applications including electric devicessuch as a cell phone, a personal computer, a video camera, an opticaldisk player, a display and a projector; automobiles; welfare devicessuch as a wheel chair; and leisure goods such as fishing goods and abicycle.

It is preferable that a scrap produced during injection molding afterfeeding the chips to the injection-molding machine is heat-molten in thepresence of a flux and then cooled to produce an ingot made of amagnesium alloy. Such a manufacturing process can provide an ingot witha reduced carbon content.

Examples of a scrap produced during injection molding include alloyssolidified inside of the injection-molding machine such as a sprue, arunner and an overflow unit, and non-standard molded articles.

The scrap is charged in a melting furnace to be molten. Here, the scrapis preferably charged in a pre-heated melting furnace. Furthermore, amolten-metal temperature is preferably adjusted to 600 to 750° C.

There are no particular restrictions to timing of adding a flux to ascrap, but it is preferably added after the scrap charged in a meltingfurnace has been molten. After a flux is added, a molten metal ispreferably refined by stirring. A refining temperature is preferably 600to 750° C. and a refining time is preferably 3 to 300 min.

There are no particular restrictions to a flux used in a manufacturingprocess for an ingot of the present invention, and those commonly usedfor refinement of a magnesium alloy can be used. An example is a fluxcontaining a halide of a metal belonging to Group 1 or 2 in the periodictable as a main component. The term “main component” as used hereinmeans a component contained in a content of generally 50% by weight ormore, preferably 80% by weight or more. The metal halide is preferablyat least one selected from magnesium chloride, calcium chloride, bariumchloride, potassium chloride, sodium chloride and calcium fluoride. Theamount of a flux to be added is preferably 0.3 to 45 parts by weight to100 parts by weight of a scrap.

A molten metal after refinement is preferably allowed to stand. Asettling temperature is preferably 600 to 750° C. and a settling time ispreferably 3 to 300 min. A clean part in the upper layer in the moltenmetal after refinement is cast in a mold and then cooled to give aningot.

A ratio (C₂/C₁) of a carbon content C₂ (% by weight) in the ingot to acarbon content C₁ (% by weight) in the scrap is preferably 0.1 or less,more preferably 0.06 or less.

It is generally difficult to remove carbon from a molten metal aftermelting of a scrap. For example, a carbon content is high in an ingotproduced by heat-melting a scrap wherein carbonized materials adhere toits surface and then cooling it. Thus, a molded article produced fromsuch an ingot has insufficient functions such as corrosion resistance.Carbon in such an ingot is not dispersed, so that properties such ascorrosion resistance would be deteriorated. Thus, for example, when ascrap having carbonated materials on its surface is regenerated into aningot, it is necessary to remove the carbonized materials beforemelting, leading to a high cost, and the carbonated materials cannot beadequately removed. In contrast, a scrap produced during molding thechips for injection molding of the present invention can be generatedinto an ingot having a low carbon content by a convenient method asdescribed above. Chips produced from such an ingot has a low carboncontent, so that it can be used in combination with chips free fromcarbon and can be used after being again coated with carbon powder withhigher recyclability. Furthermore, a molded article produced from aningot of the present invention exhibits good corrosion resistance andexcellent mechanical properties.

EXAMPLES

The present invention will be described with reference to Examples.

Tensile Test

A tensile test was conducted using a universal material testing machine“3382 Floor Model Testing System” from Instron Japan Company Ltd. A testpiece was a plate-type molded article with a thickness of 2 mm which hasa parallel portion with a width of 20 mm and a length of 60 mm in thecenter and grips at both ends. The test piece was prepared by injectionmolding using a mold for forming a test piece which has a shapecorresponding to that of the test piece. FIG. 1 shows an exterior photoof the test piece in the tensile test and an exterior photo of thetesting machine in which the test piece is set. Measurement wasconducted at a tension rate of 5 mm/min.

Bending Test

A bending test was conducted using a universal material testing machine“3382 Floor Model Testing System” from Instron Japan Company Ltd. A testpiece used in the bending test was a plate having a width of 20 mm, alength of 70 mm and a thickness of 2 mm prepared by partly cutting thegrips in the molded article formed by using a mold for forming a testpiece for the tensile test. FIG. 2 shows an exterior photo of a testingmachine in which the test piece was set in the bending test. A distancebetween two supports was set to 60 mm. A test was conducted by pushingdown a former at a rate of 5 mm/min. The test was terminated either whenthe test piece was broken or when a displacement of the former reached20 mm.

Element Mapping

Element mapping in the surface of a molded article was conducted byusing an X-ray microanalyzer “JXA-8500FS” from JEOL Ltd. An accelerationvoltage and a sample irradiation current were set to 15 kV and 1×10⁻⁸ A,respectively for measurement.

Determination of a Chemical Composition

A chemical composition of a molded article was determined using anoptical emission spectrometer “PDA-7000” from Shimadzu Corporation. Adiameter of a measuring spot was 5 mm. However, a carbon content wasmeasured as described below.

Measurement of a Carbon Content

A carbon content in a molded article was measured by using acarbon/sulfur analyzer “EMIA-920V” from HORIBA Ltd. Measurement wasconducted in accordance with JIS Z2615 “General rules for determinationof carbon in metallic materials” (infrared absorption spectrometry(integration)).

Microscopy of a Cross-Section

A molded article was cut vertically to a flow direction of a moltenmetal. After the piece obtained was embedded in a resin, the cut surfacewas polished. The cross section after polishing was observed using alight microscope.

Example 1

An ingot made of AZ91D (specifications; Al: 8.5 to 9.5% by weight, Zn:0.45 to 0.9% by weight, Mn: 0.17 to 0.4% by weight, Be: 0.0008 to0.0012% by weight, Si: 0.05% by weight or less, Fe: 0.004% by weight orless, Cu: 0.025% by weight or less, Ni: 0.001% by weight or less,balance: Mg) was cut into cylindrical magnesium alloy chips with aradius of about 0.5 mm and a length of about 4 mm. 100 kg of themagnesium alloy chips obtained and 100 g of carbon black (furnace black“#30” from Mitsubishi Chemical Corporation, average primary particlediameter: 30 nm, DBP absorption: 113 mL/100 g) were separatelyintroduced in a V-type mixer, and mixed at a rotation number of 30r.p.m. for 20 min to give chips for injection molding in which thesurfaces of the magnesium alloy chips were coated with carbon black.Here, the chips for injection molding obtained were visually observed,showing that the surfaces of the chips were substantially homogeneouslycoated with carbon black. The chips for injection molding obtained wereintroduced in an injection-molding machine for thixomolding (“JSWJLM220-MG” from The Japan Steel Works, Ltd.) and injection-molded.During injection-molding, a melting temperature and a mold temperaturewere set to 610° C. and 225° C., respectively. The mold used was a moldfor producing a test piece for a tensile test. Thus, a plate-type moldedarticle with a thickness of 2 mm which had a parallel portion with awidth of 20 mm and a length of 60 mm in the center and grips at bothends was produced. The molded article obtained had an aluminum contentof 8.9% by weight, a zinc content of 0.68% by weight, a manganesecontent of 0.26% by weight, a beryllium content of 0.0011% by weight, aniron content of 0.002% by weight, a copper content of 0.003% by weight,a nickel content of 0.001% by weight and a carbon content of 0.085% byweight. FIG. 3 shows a microgram of a cross section of a molded articlecut in a direction vertical to a flowing direction of a molten metal. Asshown in FIG. 3, there were no large cavities in the molded article.

A tensile test and a bending test were conducted for the molded articleobtained. For each test, a plurality of samples were used. FIG. 4 showsa relationship between a displacement and a load at break of the testpiece as determined by the tensile test. FIG. 5 shows a relationshipbetween a displacement and a load at break of the test piece asdetermined by the bending test. When a sample was not broken at the endof the bending test (a displacement is 20 mm), a load at the end of thetest is given. FIG. 10 shows a 0.2% proof stress as determined by thetensile test.

Element mapping was conducted for the surface of the molded articleobtained. FIG. 6 shows an elemental map of an area containing a complexof aluminum and carbon.

Distribution of a content of each of aluminum and zinc in the surface ofthe molded article were determined. FIG. 7 shows measurement points indetermination of content distribution of each of aluminum and zinc inthe molded article. FIG. 8 shows content distribution of aluminum in amolded article while FIG. 9 shows content distribution of zinc. Thenumber of sample was three for each.

Comparative Example 1

A molded article was produced using chips for injection molding whichwas not coated with carbon black. A molded article was produced asdescribed in Example 1, except that carbon black coating was notconducted. The molded article obtained had an aluminum content of 9.2%by weight, a zinc content of 0.78% by weight, a manganese content of0.25% by weight, a beryllium content of 0.0010% by weight, an ironcontent of 0.002% by weight, a copper content of 0.004% by weight and anickel content of 0.001% by weight. A carbon content was below adetection limit (0.0001% by weight). FIG. 3 shows a microgram of a crosssection of the molded article cut in a direction vertical to a flowingdirection of the molten metal. As shown in FIG. 3, relatively largercavities were observed in the molded article.

A tensile test and a bending test for the molded article were conductedas described in Example 1. FIG. 4 shows a relationship between adisplacement and a load at break of the test piece as determined by thetensile test. FIG. 5 shows a relationship between a displacement and aload at break of the test piece as determined by the bending test. FIG.10 shows a 0.2% proof stress as determined by the tensile test.Furthermore, distribution of a content of each of aluminum and zinc inthe surface of the molded article was determined as described inExample 1. FIG. 8 shows relationship between a measurement point and analuminum content, while FIG. 9 shows a relationship between ameasurement point and a zinc content.

As shown in FIG. 4, the molded article in Example 1 produced by themanufacturing process of the present invention had excellent tensilestrength. Furthermore, variation of tensile strength between samples wassmall. In contrast, the molded article in Comparative Example 1 producedusing the chips for injection molding which were not coated with carbonblack exhibited large variation of tensile strength between samples.Furthermore, as shown in FIG. 5, the molded article in Example 1 hadexcellent bending properties. In the test, any of the measured samples(four) was not broken at the maximum displacement (20 mm). In contrast,the molded article in Comparative Example 1 was broken even at a smalldisplacement and variation of bending properties between samples waslarge.

Element mapping of the molded article in Example 1 was conducted, and acomplex of aluminum and carbon as shown in FIG. 6 was observed. Such acomplex was substantially homogeneously dispersed in the surface of themolded article.

As shown in FIGS. 8 and 9, the molded article in Example 1 had lowerlevel of segregation of aluminum (FIG. 8) and zinc (FIG. 9) than themolded article in Comparative Example 1.

Examples 2 and 3

A molded article was produced as described in Example 1, except that adifferent type of carbon black was used. In Example 2, carbon black“#45L” from Mitsubishi Chemical. Corporation (average primary particlediameter: 24 nm and DBP absorption: 53 mL/100 g) was used. In Example 3,carbon black “#3050B” from Mitsubishi Chemical Corporation (averageprimary particle diameter: 50 nm and DBP absorption 175 mL/100 g) wasused. For the molded articles obtained, a tensile test was conducted asdescribed in Example 1. FIG. 10 shows a 0.2% proof stress as determinedby the tensile test.

Example 4

An ingot was produced using a scrap formed during injection-moldingchips introduced in an injection-molding machine as described inExample 1. After the injection molding, 100 kg of an alloy (carboncontent: 0.16% by weight) solidified in a sprue in the Injection-moldingmachine was placed in a pre-heated melting furnace. A temperature wasregulated to make a temperature of the molten metal 650 to 700° C. Afterthe alloy introduced was completely molten, 2 kg of a flux (Dow 310:MgCl₂ 50 parts by weight, KCl₂ 20 parts by weight, CaF₂ 15 parts byweight and MgO 15 parts by weight.) was added to the molten metal. Afterstirring for 30 min, the molten metal was allowed to stand for 30 min. Aclean part in the upper layer in the molten metal was cast in a mold andthen cooled to give an ingot. A carbon content in the ingot was 0.003%by weight. The molded article produced from chips obtained by cuttingthe ingot had comparable corrosion resistance and mechanical propertiesto those in the molded article in Comparative Example 1.

1. Chips for injection molding wherein the surfaces of chips made of analuminum-containing magnesium alloy are coated with carbon powder. 2.The chips for injection molding as claimed in claim 1, wherein a contentof said carbon powder is 0.01 to 3% by weight.
 3. The chips forinjection molding as claimed in claim 1, wherein said carbon powder iscarbon black.
 4. The chips for injection molding as claimed in claim 3,wherein said carbon black has an average primary particle diameter offrom 5 to 100 nm and a DBP absorption of from 40 to 200 mL/100 g.
 5. Aprocess for manufacturing the chips for injection molding as claimed inclaim 1, comprising mixing chips made of an aluminum-containingmagnesium alloy with said carbon powder.
 6. A process for manufacturinga molded article made of a magnesium alloy, comprising charging thechips for injection molding as claimed in claim 1 in aninjection-molding machine and then injection-molding the chips.
 7. Theprocess for manufacturing a molded article as claimed in claim 6,wherein in said molded article, a complex of aluminum and carbon isdispersed in a magnesium matrix.
 8. A process for manufacturing an ingotmade of a magnesium alloy, comprising heat-melting a scrap formed duringinjection-molding of the chips for injection molding as claimed in claim1 in the presence of a flux and then cooling it.
 9. The process formanufacturing an ingot as claimed in claim 8, wherein a ratio (C₂/C₁) ofa carbon content C₂ (% by weight) in said ingot to a carbon content C₁(% by weight) in said scrap is 0.1 or less.